I got some cheap Micromark 'easywire' 3 wire PIR sensors off ebay, and rigged them up to a 1 transistor circuit driving a relay, which was then connected to a much more complex burglar alarm system that I borrowed the design of from 'Everyday practical electronics'. I think I ran it all off a 9V regulator.

If I can find my circuit scribbles I'll draw it up neatly and put it here.

you can buy those PIR beams for shop doorways etc, they are ded cheep.... from ebay, when the beam is broken they say "Bing Bong!"They use Batteries of course and how long they last could be an issueOr these.. Hallway Alarm

I use those little wireless doorbells for our garden gate and garage door, simply wired a door contact across the button and arranged that the magnet swiped across the contact rather than holding it closed in any position, the receiver can travel anywhere in the house.. The batteries in the receiver last well over a year and only one transmitter so far has needed a battery (twice) in about 6 years!